[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #8 from 康桓瑋  ---
(In reply to Jonathan Wakely from comment #4)
> Maybe you could legally do:
> 
> using difference_type = iterator_t>;
> 
> but maybe just don't do that. If things break when you do dumb things, don't
> do those things.

A more uncontrived example would be:

  std::vector v(10);
  auto r = std::views::iota((unsigned __int128)0) | 
std::views::transform([&](auto i) -> auto& { return v[i]; });
  auto s = std::format_to_n(r.begin(), 5, "{}", "");

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

Jonathan Wakely  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |FIXED

--- Comment #7 from Jonathan Wakely  ---
Oh yes, I wonder why I bothered handling it there :-)

It should be fixed now, thanks for the report.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #6 from CVS Commits  ---
The master branch has been updated by Jonathan Wakely :

https://gcc.gnu.org/g:9cce91a63dcc40cb39e68f6a771e891e51c51946

commit r13-4527-g9cce91a63dcc40cb39e68f6a771e891e51c51946
Author: Jonathan Wakely 
Date:   Mon Dec 5 21:38:53 2022 +

libstdc++: Add casts for integer-like difference type [PR107871]

libstdc++-v3/ChangeLog:

PR libstdc++/107871
* include/std/format (_Iter_sink::_M_overflow): Add cast to
size_t.
(_Iter_sink::_M_make_span): Use
typedef instead of decltype.
* testsuite/std/format/functions/107871.cc: New test.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #5 from 康桓瑋  ---
(In reply to Jonathan Wakely from comment #4)
> Maybe you could legally do:
> 
> using difference_type = iterator_t>;
> 
> but maybe just don't do that. If things break when you do dumb things, don't
> do those things.

I would never do that.

However, I see that you seem to have considered this issue elsewhere, in
format#L2561:

if constexpr (!is_integral_v
|| sizeof(__n) > sizeof(size_t))
  {
// __int128 or __detail::__max_diff_type
auto __m = (decltype(__n))(size_t)-1;
if (__n > __m)
  __n = __m;
  }

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #4 from Jonathan Wakely  ---
Maybe you could legally do:

using difference_type = iterator_t>;

but maybe just don't do that. If things break when you do dumb things, don't do
those things.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #3 from Jonathan Wakely  ---
No. They are only allowed to be implementation-defined types, not
program-defined types.

I can add the casts to make this work, but I don't think it's a real problem
that can occur in valid programs.

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #2 from 康桓瑋  ---
This is just an example. So, are users not allowed to define integer-like class
types?

[Bug libstdc++/107871] _Iter_sink:: _M_overflow missing some difference type casting

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107871

--- Comment #1 from Jonathan Wakely  ---
This isn't valid code, you can't use max diff type for your own types. It can
only be used by the library for iota_view, but that doesn't have output
iterators.